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IDS Forum

Re: Manually create indexes for constraints or....

Posted By: Art Kagel
Date: Tuesday, 30 August 2011, at 12:25 p.m.

In Response To: Re: Manually create indexes for constraints or no (Fernando Nunes)

Fernando, you said "in 11.70 you can avoid the automatically index
creation". I'd like to clarify:

In 11.70 you can create a FOREIGN KEY constraint without a supporting
index. The index is mandatory if the constraint will include CASCADING
DELETE and is absolutely needed (though the engine has no way to enforce the
requirement) if you will be manually deleting or modifying any of the key
values in the parent or independent table to which the foreign key refers
(supported in some other RDBMS's as CASCADING UPDATEs).

I don't agree that creating foreign keys without an index is "not a good
idea". If you think through the processing involved, for foreign keys
referencing lookup tables if users never update or delete the dependent key
values and cascading delete is not defined, then the index on the dependent
table's foreign key is rarely used. The exception is parent child
relationships where queries will be driven by filters on the parent table
and the foreign key index is needed as a join index to locate child table
rows. In this case only is the foreign key constraint index required for
efficient processing.

My point is that for foreign key relationships that refer to a lookup or
code translation table, the foreign key index on the dependent table is
NEVER used, not for checking key validity, for joining, or for any other


Art S. Kagel
Advanced DataTools (

Disclaimer: Please keep in mind that my own opinions are my own opinions and
do not reflect on my employer, Advanced DataTools, the IIUG, nor any other
organization with which I am associated either explicitly, implicitly, or by
inference. Neither do those opinions reflect those of other individuals
affiliated with any entity with which I am affiliated nor those of the
entities themselves.

On Tue, Aug 30, 2011 at 12:10 PM, Fernando Nunes <>wrote:

> Hello Carlos,
> I'm not sure if I can tak "on behalf" off the "generally assumed as the
> best
> practice"....
> But in any case, I think it's nice to manually create the index not only
> because of naming conventions, but also because you can choose where to put
> the index (and some customers like to split data and indexes into different
> dbspaces).
> So, in practice, if you don't care about the naming conventions and you
> don't mind your indexes go into the same dbspace as the table, letting
> Informix do the job would be enough.
> But I think there are good reasons to create them manually. Also, from a
> very personal perspective, when I find this behavior in a customer It is
> clearer that they really know what they're doing :)
> Also note that in 11.70 you can avoid the automatically index creation
> (although in most cases this is not a good idea).
> Regards/Cumprimentos.
> On Tue, Aug 30, 2011 at 5:01 PM, CARLOS ALVES
> <>wrote:
> > Hello to all,
> >
> > I work in a software development department where it has been assumed for
> > several years that when creating constraint objects (primary, reference
> or
> > check) on our development/production Informix databases, one should
> always
> > manually create before the respective index. This allows for following
> some
> > naming rules, like the index to have the same name as the constraint. But
> > if
> > no index is created, Informix always creates one automatically for the
> > given
> > columns of the constraint (even though it has that numerical
> nomenclature).
> >
> > So I'd like to know what is generally assumed as the best practice:
> should
> > one
> > manually create the index or from a strictly practical point of view, is
> > the
> > one automatically created by Informix simply enough?
> >
> > Thank you very much for your kind help.
> >
> >
> >
> >
> *******************************************************************************
> > Forum Note: Use "Reply" to post a response in the discussion forum.
> >
> >
> --
> Fernando Nunes
> Portugal
> My email works... but I don't check it frequently...
> --002215b03466cdb8aa04abbb3faf
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.


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